Why would you change the copyright forward. A copyright 1994 means "This content exists since 1994, if you copied after that, there is proof I have made this before you"
In US copyright law, there are three dates that control copyright: 120 years from creation (first fixation), 95 years from first publication, or life + 70 years. If any one of those terms expire [1], the work moves into the public domain.
Affixing an earlier copyright date affords no advantages. Pre-1976 copyright reform, flubbing the formalities means you lose copyright entirely. After that reform, you still keep copyright, but instead you lose out on some ability to recuperate losses, and you give your opponent extra legal ammunition in any copyright infringement cases.
[1] Strictly speaking, life + 70 years actually rules when the author is determined. If you read the law carefully, once the fixation/publication threshold is crossed, it is now the burden of proof of the copyright owner to demonstrate that life + 70 hasn't happened yet, and furthermore to demonstrate that the alleged infringer failed to do due diligence to determine the death date of the author. In practice, it could end up really being a lesser-of-all-three situation, but we aren't going to get any case law on this until 2074 at the earliest, since life + 70 doesn't start until 1978.
Not in most jurisdictions, no. What's your country where copyright renewal is still a thing? (no judgement here, despite all my hatred for copyright i'd rather have copyright renewal than copyright until 50y after death of all editors/compositors/authors as we have in Europe)
Moreover, under United States law, new editions of a preexisting work are considered "derivative works" and in these cases "the editorial revisions, annotations, elaborations, or other modifications represent, as a whole, an original work." [1]
SIGNIFICANTLY, this U.S. government source explicitly mentions the following as cases of derivative works which allow for a new copyright:
* A new version of an existing computer program
* A revision of a website
The new copyright on the derivative work "covers only the additions, changes, or other new material appearing for the first time in the work."
In other words, by creating a derivative work via updating an existing computer program or modifying a website, you absolutely gain a new copyright on any new material and you absolutely can and should update the copyright date under U.S. law.
People urging the contrary, at least insofar as they are talking about the United States or governments with similar copyright regimes are dead wrong, and jcranmer and garmaine are right and unfairly downvoted.
The original copyright is not extended, no, unless i have some serious legal misunderstandings. For example, translations for a book will have a newer copyright, but the original text's copyright will not be extended by receiving a new translation.
It would be slightly more accurate to say that you receive a new copyright in any new material in the work, not that your copyright is extended on the old material. Under U.S. law at least, a significant alteration of a work results in a derivative work under law, and you are correct to say that it is entirely reasonable to place an updated copyright date on that work.